One of sensors conventionally used in automobiles is an oxygen sensor which is disposed in an exhaust passage of an internal combustion engine (automotive engine) so as to detect the concentration of oxygen contained in exhaust gas. This oxygen sensor detects not only the oxygen concentration of exhaust gas but also the air-fuel ratio of exhaust gas, by making use of a phenomenon that the magnitude of current flowing through a sensor element changes in accordance with the oxygen concentration of exhaust gas. A sensor control apparatus that controls drive of this oxygen sensor has a function of controlling the supply of electricity to the sensor element, converting current flowing through the sensor element to a voltage, and outputting the voltage to an electronic control unit (ECU). The ECU obtains the oxygen concentration and/or air-fuel ratio of exhaust gas on the basis of an output from the sensor control apparatus. In the ECU, the obtained oxygen concentration and/or air-fuel ratio of exhaust gas is utilized for air-fuel ratio feedback control such as adjustment of fuel injection amount.
An oxygen sensor mounted on an automobile has a possibility of its output shifting under a given condition if the characteristic of the sensor changes as a result of, for example, deterioration with time or deterioration of permanent properties. Therefore, there has been known a technique of correcting a change in the characteristic of an oxygen sensor on the basis of a sensor output in an air. For example, there has been disclosed an exhaust gas concentration detection method and an apparatus therefor in which air is supplied to a gas sensor, and the zero point of the detection output of the gas sensor is calibrated on the basis of the detection output of the gas sensor exposed to air (for example, see Patent Document 1). Further, there has been disclosed an air-fuel-ratio detection apparatus which detects the humidity of air, corrects an atmospheric condition output in accordance with the humidity, and employs the corrected output as an air calibration value, which accurately corresponds to the oxygen partial pressure of air, whereby the sensor characteristic is corrected accurately (for example, see Patent Document 2).